1. Field of the Invention
The present invention generally relates to systems for calibration of combined oxygen and combustibles analyzers and particularly to automatic control systems for calibrating and monitoring the signals of combined oxygen and combustibles analyzers.
2. Description of Prior Art
Many known volatile atmosphere processes, such as boiler furances and coal pulverizers, require constant monitoring to detect potentially flammable or explosive conditions and alert an operator to such conditions.
Presently, several systems are being used to monitor potentially dangerous coal pulverizer atmospheres. Thermocouples and infrared gas analyzers are two systems used to monitor pulverizers. A shield is needed to protect these systems from the corrosive coal particles in the coal pulverizer. The shield reduces heat conduction to the thermocouple system thereby reducing response time. Infrared gas analyzers need to condition the sample gas also decreasing response time and sensitivity.
A more reliable method used to monitor volatile atmospheres in coal pulverizers is to provide signals indicative of either oxygen, CO, or both. A certain quantity of each of these elements in a volatile atmosphere becomes indicative as to the presence of a hazardous condition. Combination oxygen/combustibles analyzers such as the Bailey Controls Model OL230 are used to provide such signals or combinations of them.
These analyzers must be calibrated frequently to make sure that they are providing an accurate measurement of the forementioned elements in the volatile atmosphere. Presently, the calibration procedures for such analyzers are not automated with the entire analyzer control system.
The present method for calibrating these analyzers is manual. The operator manually introduces a test gas to the analyzer, maintaining the air pressure as if the analyzer was in operation. While maintaining the test gas pressure, the calibration potentiometers are manually adjusted and test voltage outputs are monitored through hand held voltmeters until the desired calibrated outputs are attained.
A known method for calibrating a combustibles signal consists of introducing a test gas to the analyzer and maintaining the sample air pressure as in the manual calibration of the oxygen signal. The zero and span (max. scale value) values from the test gas are adjusted by an operator. The incoming sample signal is then calibrated according to these adjusted values.
The known calibration systems for combination oxygen/combustibles analyzers are also manual. This inhibits the calibration of a plurality of analyzers by one operator. The oxygen calibration operation requires at least fifteen minutes of operator time. This manual operation introduces the possibility of operator error and does not allow for repeatability of zero and span values. Further, the calibration of both combustibles and oxygen signals is not coordinated and thus hinders a plurality of analyzers from being calibrated concurrently.
Since the oxygen signal can drift significantly out of calibration before an operator becomes aware of the need for recalibration, a significant error with the oxygen signal is generated by the drift before it is detected and corrected.
Thus, it is seen that an accurate and reliable automatic calibration system was required for periodically calibrating the oxygen signal of oxygen/combustibles analyzers and coordinating it into a total control system for monitor/alarm/calibration of the entire safety monitoring unit.